The ability to classify vehicles quickly and efficiently is needed in a number of transportation and security applications. Laser scanner sensors have been employed for vehicle classification. In general, laser scanners are time-of-flight measuring devices that emit extremely short pulses of light towards objects. These devices can detect the part of the light that is reflected from the objects, measure the time difference between emission and reflection, and derive a distance to the object. The operation is repeated for a varying set of angles. Scanners are usually placed overhead, with the laser curtain facing downwards, and perpendicular to the traffic direction, so that the scanner plane cuts the vehicle in transversal “slices”.
These scanners thus provide means for vehicle classification. Although other sensors such as electromagnetic loop detectors are also used for this purpose, these usually involve road works and cannot be placed in sensitive locations such as bridges. Laser scanners are a common type of equipment for vehicle classification, especially for determining the vehicle type in tolls. However, this is usually achieved by simple solutions such as extracting a few high-level features and classifying them (sometimes with simple rules), or matching the raw laser profiles.
As an example, some systems have been developed for vehicle classification, which extract the following features from a laser scan: width, height, length, number of vehicle units, height of the first unit. Using this approach, vehicles can be classified using simple rules comparing feature values with pre-specified thresholds. While such an approach might provide initial satisfactory performance, it is limited in scope and efficiency, requires intense human labor and expertise, and lacks learning classification capabilities.